1. Try this. You will find it is wrong when you said m// returns undef for non-integers, in fact the return value is empty string.

   $a = 123;
   print $a =~ m/^\d+$/, "\n";
   
   $a = "abc";
   $ret = ($a =~ m/^\d+$/);
   if (defined($ret)) {
       print "=$ret=\n";#print out ==
   } else {
       print "undef";#not going this loop
   }
   [download]

2. Also try this, which tells you what is "true", what is "false" in Perl:

   sub true_false {
       if (shift()) {
           print "true\n";
       } else {
           print "false\n";
       }
   }
   
   true_false("a");
   true_false(1);
   true_false("");
   true_false(0);
   
   if (undef) { #I don't want to use true_false for this case, so it is g
   +uranteed that shift() does not play any magic
       print "true\n";
   } else {
       print "false\n";
   }
   [download]

3. Go perlop, and look under m//, there is a clear answer for the return value from m//, under different situations. Not just the context, but also the modifiers changes the meaning of the return value from m//

i am quite aware of what values are ture and what are false. explicitly, undef and empty string are false values. and the perlop says...

       m/PATTERN/cgimosx
       /PATTERN/cgimosx
               Searches a string for a pattern match, and in
               scalar context returns true if it succeeds, false
               if it fails. ...

...it doesn't say anything about what the actual values are.

i was asking why is that to express success number 1 is returned, yet a non-number is returned on failure. is it an implementation detail/decision such that empty string is easier/faster to return than the number 0?

It doesn't return 1 for success. It returns the number of matches (or substitutions for s/// or tr///). If you're not using the g option, it stops on the first match and returns 1. I'm not really up on perl internals but I would guess undef is represented as a null pointer whereas a zero would require creation of an SV.

---
print map { my ($m)=1<<hex($_)&11?' ':'';
$m.=substr('AHJPacehklnorstu',hex($_),1) }
split //,'2fde0abe76c36c914586c';
[download]

m// returns true or false in scalar context. Perl defines:

NAME	Value as String	Value as Number	Perl Internal Variable
true	"1"	1	PL_sv_yes
false	""	0	PL_sv_no

For evidence, see the source code module perl.c where PL_sv_yes and PL_sv_no are initialized. Also see the pp_match code in pp_hot.c where RETPUSHYES and RETPUSHNO are used to return a boolean value when invoked in scalar context without /g.

Let me show you a piece of code first:

$_ = 123;
print 1 + m/^\d+$/, "\n"; # get 2
print "1" . m/^\d+$/, "\n"; #get 11
[download]

What does this piece of code tell us? It clearly tells us that, it is meaningless to ask the question whether m// returned a string or a number. The answer is only meaningful, when you use the returned value for a PURPOSE, either as a number, or as a string.

This is the flexibility we get from Perl.

Something not related, yet related:

Before you look at it, the Schrodinger's cat is both dead and alive. Dead or alive only becomes determined after you looked at the cat.

The "undefined value" does test false?

sub if_integer{ 
    return defined $_[0] && $_[0] =~ m/^\d+$/; 
}

if_integer($_) and print $_, ' is an integer', $/ 
    for -1, 0, 1, 2, 2.2, 1E0, '1E2', 'A', '123', '123 fred'

0  is an integer
1  is an integer
2  is an integer
1  is an integer
123  is an integer
[download]

So the logic of the test works fine.

Of course your regex is a little primitive as it excludes all negative integers, as well as those represent in exponential form, '1E0' for example.

yes undef is false but causes problem if used as is when some non undef value is expected. thus the addition of '|| 0' in the function.